This invention relates to a flexible multilayer heat sealable general purpose packaging film and to a method of forming the same.
In the packaging of certain types of foods, for example, loose products such as cookies, potato chips, and the like, it is common practice to employ a multilayer film having two or more polymeric layers wherein one of the layers is known to be an effective heat seal layer. In the packaging process, a supply of such a multilayer film can be shaped into a tube in a vertical form and fill machine. Marginal regions of the heat seal layer are brought into face to face relationship and heat sealed together. Thereafter, the packaging machine automatically forms a heat seal and makes a horizontal severance across the bottom of the bag; product is dispensed into the open end of the tube and thereafter a second horizontal seal is effected across the tube with a simultaneous severing through the tube to result in a product packaged in a tube, heat sealed at both ends and along one seam at right angles to the end seals. While the food or other product is being dispensed into the package, air is also present in the package and this air assists in protecting and cushioning the product during subsequent shipment of the finished packages. During shipment of the product, particularly with larger size bags, e.g., those containing 16 ounces of product, the bags have a tendency to split or burst at the end seals.
A multilayered wrapping film of the type having the above mentioned utility is described in U.S. Pat. No. 4,214,039, the subject matter of which is, in its entirety, incorporated by reference herein. This patent describes a packaging film structure comprising a polypropylene film substrate having a heat sealable vinylidene chloride polymer containing at least 50% by weight of vinylidene chloride. In between the polypropylene film and the vinylidene chloride polymer is a primer coat which consists of the reaction product of an acidified aminoethylated vinyl polymer and an epoxy resin. This primer coat enhances the bond strength between the polypropylene and the heat sealable vinylidene chloride polymer. While this packaging material is effective for the packaging of comparatively small quantities of product, there exists a need to increase its seal strength when used for the packaging of comparatively large quantities of product. Particularly there is a need to increase the seal strength between the polypropylene layer and the primer layer.